Solving a problem in physics section 3 point 84 mechani

84. 4 kg mass flywheel rotates freely around a horizontal axis passing through its center, making the 720 rev / min. The mass of the flywheel can be considered distributed over its rim radius of 40 cm. After 30 s under the influence of drag torque flywheel stopped. Find braking torque M and the number of turns N, which makes the flywheel to a stop.